1. Field of the Invention
The invention relates in general to escalators, and more specifically to escalators which have two or more drive units coupled to drive the endless belt to which the escalator steps are attached.
2. Description of the Prior Art
U.S. Pat. Nos. 3,677,388 and 3,707,220, which are assigned to the same assignee as the present application, disclose a modular drive unit, and modular passenger conveyor construction, respectively. The modular drive units are inserted into selected modules of the passenger conveyor, on the inclined portion thereof, according to vertical rise. For example, for a 48 inch wide escalator, a single drive unit is used up to 20 feet of rise, with an additional drive unit being added for each multiple of 20 feet of rise thereafter.
When more than one drive unit is used, the drive units must be precisely spaced from one another such that each supports the load of the running gear, i.e., steps, step axles and step links, between itself and the next lower drive unit on the incline. To illustrate the criticality of drive unit spacing, assume a three drive unit escalator. If the drive units are initially spaced such that each supports all of the load between it and the next lower drive unit, i.e., all step links are in tension, and then the middle drive unit is moved down the incline by only 0.125 inch, then all of the load between the two lower drive units will be imposed on the uppermost drive unit, and the middle drive unit would support no mechanical load.
In order to assure that such a condition does not exist, the recommended adjustment of the drive units is to move each of the drive units which are located below the uppermost drive unit slightly up the incline from the location which tensions every step link between the drives, so that one to two step links are in compression as they leave each of the lower drive units.
This recommended adjustment, however, is difficult and time consuming to achieve. Calculation methods all relate to a static system, and are inaccurate due to variations due to the escalator brake, the hand rail drives, and the system friction. Further, once satisfactory drive spacing is initially achieved, the procedure must be repeated periodically in order to accommodate wear of the step link bushings, step links, and step axles.